Steam cracking, which is the thermal cracking of hydrocarbons in the presence of steam, is used commercially in large scale industrial units to produce ethylene and to a lesser extent propylene. These pyrolysis units are often charged a naphtha boiling range feed stream. The typical petroleum derived naphtha contains a wide variety of different hydrocarbon types including normal paraffins, branched paraffins, olefins, naphthenes, benzene, and alkyl aromatics. It is known in the art that paraffins are the most easily cracked and provide the highest yield of ethylene and that some compounds such as benzene are relatively refractory to the typical cracking conditions. It is also known that cracking normal paraffins results in a higher product yield than cracking iso-paraffins. A paper entitled “Separation of Normal Paraffins from Isoparaffins” presented by I. A. Reddoch, et al, at the Eleventh Australian Conference on Chemical Engineering, Brisbane, Sep. 4-7, 1983 discloses that the ethylene yield of a cracking unit can be increased if it is charged a C5 to C9 stream of normal paraffins rather than a typical C5 to C9 natural gasoline.
The separation of the myriad components of a petroleum naphtha into specific structural types by fractional distillation, a form of fractionation, is expensive and complicated and any attempt to improve the character of the naphtha as a steam cracking feed employs other means which act on a class of structural types, such as extraction.
The benefits of separating the various classes of hydrocarbons in petroleum fractions have led to the development of a number of different techniques which separate the hydrocarbons by type rather than individual molecular weight or volatility. For instance, various forms of liquid extraction can be used to remove aromatic hydrocarbons from a mixture of aromatic and paraffinic hydrocarbons. Adsorptive separation techniques have been developed to separate olefins from paraffins and to separate normal (straight chain) paraffins from non-normal, e.g. branch chain paraffins and aromatics. An example of such a process is described in GB 2,119,398A which employs a 5 Å zeolite having crystals larger than 5 Å to selectively adsorb straight chain hydrocarbons to the exclusion of non-straight chain hydrocarbons and sulfur compounds.
There are great economic benefits to a large scale unit if an adsorptive separation is performed in a continuous manner. U.S. Pat. No. 4,006,197 and U.S. Pat. No. 4,455,444 describe techniques for performing a continuous simulated moving bed (SMB) adsorptive separation process for the recovery of normal paraffins, which is the preferred mode of operating the adsorptive separation zone of the subject invention. U.S. Pat. No. 4,006,197 describes the fractionation of the raffinate and extract streams to recover desorbent which is reused in the process.
U.S. Pat. No. 3,291,726 describes the use of simulated moving bed technology to separate normal paraffins from a petroleum derived fraction. U.S. Pat. No. 6,407,301 describes the use of simulated moving bed technology to separate normal paraffins from non-normal hydrocarbons to generate a feed to a steam cracking zone and a feed to a catalytic reforming zone. Both references further describe that a suitable desorbent for use in the process may be provided by fractional distillation of the feedstock and the raffinate and extract removed from the adsorption zone.
Having the desorbent used in the simulated moving bed generated by fractional distillation of the feedstock often results in a desorbent that has a boiling point fairly close to that of the components of the raffinate or extract. Separation and recycle of the desorbent may require more costly equipment, such as increased stages in distillation columns, and more utilities costs associated with the larger equipment. When using simulated moving bed technology to separate normal paraffins from non-normal hydrocarbons in order to generate a feed to a steam cracking zone and a feed to a catalytic reforming zone, employing a desorbent having a boiling point diverse from that of the components of the raffinate and extract streams and not present in the feed stream results in significant cost reductions. The raffinate column and the extract column may be reduced in size and the utilities consumption may be reduced. There is no need to vaporize the desorbent and so there is a reduced utilities consumption in the raffinate and extract columns. Also, the reflux ratio is reduced thereby conserving costs. As compared to other operations, the feed stream to the simulated moving bed need not be fractionated before being introduced to the simulated moving bed, thereby eliminating the costs associated with one fractionation column.
However, most importantly, the A/Fn ratio may be in the range of 0.90 to about 0.92 which is significantly less than a standard range of from about 1.0 to about 1.2. The effect of the significantly reduced A/Fn ratio is the ability to process more material with less adsorbent thereby reducing costs.